The methods of x-ray crystallography will be used to determine the molecular structures of materials of biological interest. The interpretation of the x-ray diffraction data to yield information about molecular structure will utilize computer methods developed as part of this project. These methods will be further improved in the light of the experiences gained. The structure analysis of crystalline Satellite Tobacco Necrosis Virus will be continued with special emphasis on attempts to learn something about the structure of the viral RNA. The structure analysis will be carried to higher resolution than the present 3.8 A. It is expected that the above computer methods, exploiting non-crystallographic symmetry for phase determination, will be applied to other viruses and/or multisubunit proteins.